SUMMARY While U.S. CDC annual estimates of Chlamydia trachomatis (Ct) sexually transmitted infections (STIs) are about 3 million, global World Health Organization (WHO) estimates are >131 million. Over 61 million people are infected among the Pacific Island Countries and Territories (PICT) of the Western Pacific Ocean with a prevalence rate of ~40% among teens and young adults. These percentages reflect the fact that STIs are a major area of health disparity in the PICT as well as in other parts of the world. In the U.S., Hawaiian and other Pacific Islanders have the 3rd highest prevalence of STIs, which is 3.7 times that of Whites. In these resource- constrained regions, syndromic management of Ct is the norm. This is problematic because ~80% of females and 50% of males are asymptomatic and do not seek medical care. Transmission from these asymptomatic yet infected individuals to partners likely fuels the ongoing worldwide epidemic. Further, lack of treatment can result in serious sequelae such as pelvic inflammatory disease, infertility, ectopic pregnancy, and hemorrhagic proctitis. While the endocervix is considered the primary site of infection, female Ct rectal infections now outnumber those in the urogenital tract. Without adequate detection, the rectum, which requires 7 to 21 days of treatment with high rates of recurrence, is a potential reservoir of Ct for transmission within the host and to partners. Our unifying hypothesis is that the natural history of Ct STIs is defined by the interaction of the microbiomes, immune responses and pathogen populations of three key body sites: the vagina, endocervix and rectum. We will employ metagenome shotgun sequencing (MSS) to understand healthy, dysbiotic and Ct- associated microbiota in addition to host immune responses and Ct pathogen characteristics for a high- incidence cohort of Fijian women. This work will naturally transition to improving future Ct diagnostics that utilize metgenomic methods, and we will determine whether these data can predict protection from Ct and/or incident Ct and infection severity. With prospective samples and clinical data collected prior to and at incident Ct infection (or no Ct) from our cohort, we aim to: 1) identify taxonomic diversity, richness and abundance of DNA-based organisms in the endocervix, vagina and rectum using MSS cross-referenced to 16S sequencing at both time points; 2) quantitate immune responses in the context of the microbiota for each site, time point and clinical outcome; 3) determine microbiota/immune response profiles that correlate with incident Ct genomic strains and whether strain plays a role in clinical outcome at each site. Our research will aid in selecting optimal sites for Ct screening and designing strategies such as vaginal and/or rectal therapy with beneficial microbiota, especially for the latter site given the long and often ineffective treatment regimens. The steady global increase in Ct cases necessitates research especially among those who suffer from health disparities and are at increased risk for STIs. We have assembled a unique cohort of Fijian women with high rates of Ct (up to 38%) without which it would not be possible to study the natural history of Ct urogenital and rectal STIs.